1. Field of the Invention
The present invention relates to an improvement of a ceramic heater and particularly it is applied to a heater for the regeneration of an apparatus for collecting and purifying fine particles contained in the exhaust gas of a diesel engine or the like.
2. Description of the Related Art
For collecting and purifying fine particles contained in the exhaust gas, it has been proposed to use a heat-resistant honeycomb- or foam-like filter made of a ceramic material and to fire and burn the collected fine particles by means of a heat-resistant ceramic heater disposed on an upper surface of the filter to regenerate the filter.
In this case, rectangular plate-like heaters as shown in FIG. 8 or plate-like heaters formed in the shape of U or V as shown in FIG. 9 have been proposed. Moreover, it has been considered desirable to dispose a plurality of these heaters in a filter.
All these heaters have a common method of holding a heater, which is based on an electrode cantilever holding structure. That is, one side electrode is fixed completely to a case by welding as an earth terminal, and a plus side electrode is capable of sliding freely with an opening or closing motion of heater legs which is caused by heating and cooling of the heater.
In the U- or V-shaped heater, if the plus-side electrode in addition to the minus-side electrode is completely fixed to the case, stress is concentrated on a bent portion and the heater breaks easily. Moreover, if the heater having an electrode cantilever holding structure is installed in an engine, engine vibrations are conveyed to the heater, and the bent portion vibrates badly. Accordingly, the attached portion of the minus-side electrode to the case tends to be weak in strength.
Usually, a U- or V-shaped heater is connected at its both end electrode portions 3' to metallic lead wires for example as shown in FIG. 9 and is thereby charged electrically and heated, so with a view to protecting the electrode portions the sectional area of the electrode portions is made fairly larger than that of a heat generating portion to suppress the heat generation at the electrode portions.
However, in a conventional ceramic heater there arises an opening-closing motion of leg portions in the course of heat generation and cooling due to thermal expansion, and once the heater is fixed to a holding portion, the above motion is impeded by friction, for example, and stress is concentrated on a bent portion 2a'. This stress concentrated on a bent portion 2a' is proportional to moment of an external force exerted on the leg portion and a length (Lo) from the leg portion to the bent portion 2a'. Consequently, this shape causes the problem of stress being concentrated on the bent portion 2a', and the repeated supply of electricity results in cracking of the bent portion.
The above conventional ceramic heater also involves the problem that when it is disposed on the filter surface, it is impossible to heat the filter surface widely because the area of heat generation is small in comparison with the large space of the electrode portions.
Accordingly, one of the applicants of the present invention provided a ceramic heater (Japanese patent application No. 60-3459 from which priority is claimed in Hoshizaki et al Ser. No. 816,868 filed in January 1986) capable of preventing stress concentration during heat generation of the heater, ensuring a uniform heating property, also ensuring a large heat generating area and facilitating the handling of electrodes. The ceramic heater of the unpublished related application is a ceramic heater in which two electrode portions 35a and 35b are provided on a single straight line or curved line m.sub.5 and a heat generating portion is positioned on one side of the line, for example, as shown in FIG. 10.
The above heat generating portion comprises a closed loop portion 15 and two leg potions 25a and 25b integrally connected to the ring-like portion 15 at two connecting portions 15c and 15d which approximately bisect the closed loop portion 15. The two electrode portions 35a and 35b are each provided at fore ends of the two leg portions 25a and 25b respectively.
This ceramic heater is characterized by having a construction in which the closed loop portion 15 and the two electrode portions 35a and 35b conjointly form a triangle.
However, in even the above ceramic heater, holding method of the heater can take only an electrode cantilever holding structure. Therefore, the problem that cracks are frequent at the minus-side electrode connected to the earth occurs as well as a conventional ceramic heater.